90

What You’ll Learn■ You will explain how popula-

tions grow. ■ You will identify factors

that inhibit the growth ofpopulations.

■ You will summarize issues inhuman population growth.

Why It’s ImportantHow a population of organismsgrows is critical to the survivalof its species. A population thatgrows rapidly may run out offood or space. A population thatgrows too slowly may becomeextinct.

Population BiologyPopulation Biology

Art Wolfe/Photo Researchers

Visit to• study the entire chapter

online• access Web Links for more

information and activities onpopulation biology

• review content with theInteractive Tutor and self-check quizzes

King penguins are highly socialanimals that live and breed inlarge colonies called rookeries insome of the most isolated islandsin the subantarctic. Even thoughfew people will ever encounterthese animals, maintaining theirpopulations is important forkeeping the ecosystems in thatportion of the world healthy.

Understandingthe Photo

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Population Dynamics4.1

Principles of Population GrowthA population is a group of organisms, all of the same species, that live in

a specific area. There are populations of spruce trees, populations ofmaple trees, of bluebirds, dandelions, fruit flies, and house cats.

Every organism you can think of is a member of a population.A healthy population will grow and die at a relatively steady

rate unless it runs out of water, food, or space, or is attackedin some way by disease or predators.

Scientists study changes in populations in a variety of ways. One method involves introducing organismsinto a controlled environment with abundant resources;then watching how the organisms react. That is what is happening in Figure 4.1. Bacterial cells are placed

in a dish of sterile, nutrient-rich solution and pop-ulation growth is observed over a period of time. Through

studies such as these, scientists have been able to identifysome trends in the growth of bacterial cells.

SECTION PREVIEWObjectivesCompare and contrastexponential and linearpopulation growth.Relate the reproductivepatterns of different populations of organismsto models of populationgrowth.Predict effects of environmental factors on population growth.

Review Vocabularypopulation: a group of

organisms of the samespecies that live in aspecific area (p. 38)

New Vocabularyexponential growthcarrying capacitylife-history patterndensity-dependent factordensity-independent factor

Compare and Contrast As you read Chapter 4, compare the effects thesefactors have on population size. List some of the effects under the appropriate tab.

Populations Make the following Foldable to help you identify the main factors that affect populations.

Fold a sheet of paper in half lengthwise.

Fold in half, then fold in half again to make three folds.

Unfold and cut only the top layer along the three folds to make four tabs.

Label the tabs.

STEP 1

STEP 3

STEP 2

STEP 4

PopulationIncrease

CarryingCapacity

TheEnvironment

OtherOrganisms

4.1 POPULATION DYNAMICS 91Biophoto Associates/Photo Researchers

Figure 4.1 Ecologists can study bacterialpopulation growth in thelaboratory.

Standard 6c Students know how fluctuations in population size in anecosystem are determined by the relative rates of birth, immigration, emigration, and death.

California Standards

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92 POPULATION BIOLOGYKS Studios

Popu

latio

n si

ze

500 000

1 million

100One year

Population Growth of HousefliesFigure 4.2 Because they grow expo-nentially, populations ofhouseflies have the poten-tial for unchecked growth.Notice that the shape ofthe curve is like the letter J.

Information on bacterial cell growthmight be helpful in fighting disease.Studies of populations of larger organ-isms, such as an elk population in anational park, require methods such asthe use of radio monitors. Use theMiniLab on this page to learn onemethod of measuring growth in a fruitfly population.

How fast do populations grow?The growth of populations is unlike

the growth of pay you get from a job.Suppose your job pays $5 per hour.You know if you work for two hours,you will be paid $10; if you work forfour hours, you will be paid $20; if youwork for eight hours, you will be paid$40; and so on. If you were to plotmoney earned against your time inhours, the graph would show a steady,straight-line (linear) increase.

Populations of organisms, however,do not experience linear growth.Rather, the graph of a growing popu-lation starts out slowly, then begins toresemble a J-shaped curve, as illus-trated in a population of houseflies inFigure 4.2. The initial increase in thenumber of organisms is slow becausethe number of reproducing individu-als is small. Soon, however, the rate ofpopulation growth increases becausethe total number of individuals thatare able to reproduce has increased.

Make and Use Tables Fruit Fly Population Growth Fruit flies (Drosophilamelanogaster) are used in biological research because theyreproduce quickly and are easy to keep and count. In this activ-ity you will observe the growth of a fruit fly population as itexploits a food supply.

Procedure! Place half of a banana in an

uncovered jar and allow it to sitoutside in a warm shaded area,or put it in a warm area in yourclassroom.

@ Leave the jar for one day or untilyou have at least three fruit fliesin it. Put a cloth on top of the jarand fasten with the rubber band.

# Each day for at least three weeks,record how many adult fruit flies are alive in the jar. Put data into table form. Graph yourdata. CAUTION: Make wise choices. Do not eat the fruit.Return the flies and fruit to your teacher for disposal.

Analysis1. Observe and Infer How many fruit flies did you start

with? On what day were there the most fruit flies? Howmany were there?

2. Analyze Trends from Data Why do you think the number of fruit flies decreased?

3. Predict Trends from Data What might help the population to begin growing again after a decrease?

Fruit flies feedingon a banana

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Is growth unlimited?A J-shaped growth curve illus-

trates exponential population growth.Exponential growth means that as apopulation gets larger, it also grows ata faster rate. Exponential growthresults in unchecked growth.

What can limit growth?Can a population of organisms

grow indefinitely? Through obser-vation and population experiments,scientists have found that populationgrowth does have limits. Eventually,limiting factors, such as availabilityof food, disease, predators, or lack of space, will cause populationgrowth to slow. Under these pres-sures, the population may stabilizein an S-shaped growth curve, whichyou can see in Figure 4.3.

Carrying capacityThe number of organisms of one

species that an environment cansupport indefinitely is its carryingcapacity. When a population isdeveloping in an environment withresources, there are more birthsthan deaths and the populationincreases until the carrying capacityis reached or passed. When a popu-lation overshoots the carryingcapacity, then limiting factors maycome into effect. Deaths begin toexceed births and the populationfalls below carrying capacity. Thus,the number of organisms in a popu-lation is sometimes more than theenvironment can support and some-times less than the environment cansupport. Figure 4.4 on the next pageshows a population growth line thatmoves above and below the carryingcapacity. Many different types oforganisms can show such growthpatterns in nature.

Reproduction PatternsIn nature, animal and plant popula-

tions change in size. For example,mosquitoes are more numerous atcertain times of the year than others.Why don’t populations reach carry-ing capacity and remain stable? Toanswer this question, populationbiologists study the factor thatdetermines population growth—anorganism’s reproductive pattern,also called its life-history pattern.

A variety of population growth patterns are possible in nature. Twoextremes of these patterns are demon-strated by the population growthrates of mosquitoes and elephants.Mosquitoes exhibit a rapid life-history pattern. Elephants, like manyother large organisms, exhibit charac-teristics of the slow life-history pat-tern. Mosquitoes reproduce veryrapidly and produce many offspringin a short period of time, whereas ele-phants have a slow rate of reproduc-tion and produce relatively few youngover their lifetime.

4.1 POPULATION DYNAMICS 93

Popu

latio

n Time

Characteristics of Population Growth

Exponentialgrowth

Carryingcapacity

J curve S curve

0

Dis

ease

Spac

e

Pred

ato

rs

Foo

d

Figure 4.3 The graph above compares exponentialgrowth and growththat is influenced bylimiting factors such as disease, space, pred-ators, or food. Overtime, one or more ofthese limiting factorscan keep a populationat or below the carry-ing capacity of theenvironment.

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94 POPULATION BIOLOGYFrancois Gohier/Photo Researchers

Time

Carrying capacity

Num

ber

of o

rgan

ism

s of

one

spe

cies

Beginning growth Thepopulation increase begins slowly,as the few starting members haveoffspring.

AA

Rapid growth Thereare many organisms, eachreproducing, resulting ina faster increase in thenumber of individuals.Growth is exponential.

BB

Leveling off As the populationgrows, more organisms are usingthe existing resources. Growthslows. Overall, the graph begins to resemble the letter S.

CC

Fluctuations Thenumber of organisms tendsto rise above and fall belowthe carrying capacity due tolimiting factors.

EE

Humpback whales have a slow life-history pattern.

Population GrowthFigure 4.4When a population is in an environment unaffected byfactors such as predators, fire, or drought, and there aresufficient resources, the population increases. Ecologistshave discovered that these population increases show apattern. Whether it is a plant or animal, whether on landor in the ocean, populations grow in predictable manners.Critical Thinking Why can a population fluctuate onceit reaches carrying capacity?

Carrying capacity Theenvironment can support thismany organisms. If populationsize rises above the carryingcapacity, more organisms die thanare born. The population dropsbelow the carrying capacity.

DD

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4.1 POPULATION DYNAMICS 95(t)Blair Seitz/Photo Researchers, (b)Rod Planck/Photo Researchers

Figure 4.5Wild mustard plants taking over anabandoned field represent a specieswith a rapid life-history pattern (A).Organisms that have a slow life-historypattern, such as these pronghorn ante-lope, provide much parental care fortheir young (B). Predict Trends Whichof these organisms would bemore successful in a rapidly changing environment? Explain.

AA

BB

Rapid life-history patternsRapid life-history patterns are com-

mon among organisms from change-able or unpredictable environments.Rapid life-history organisms have asmall body size, mature rapidly, repro-duce early, and have a short life span.Populations of rapid life-history organ-isms increase rapidly, then declinewhen environmental conditions such astemperature suddenly change andbecome unsuitable for life. The smallpopulation that survives will reproduceexponentially when conditions areagain favorable. The Problem-SolvingLab on this page explores growth inbacteria, an organism with a rapid life-history pattern.

Slow life-history patternsLarge species that live in more

stable environments usually have slowlife-history patterns. Elephants, bears,whales, humans, and plants, such astrees, are long lived. The pronghornantelope shown in Figure 4.5,are slow life-history organisms.

PredictHow rapidlycan bacteriareproduce?Bacteria areexamples of rap-idly reproducingorganisms. Theyare often used inexperiments about population studies or trends.

Solve the ProblemHere are some facts regarding unchecked bacterial reproduction:1. A single bacterium can reproduce to yield two bacteria

under ideal conditions every 20 minutes.2. Ideal conditions for bacterial reproduction include proper

temperature, unlimited food, space to grow, and disper-sion of waste materials.

Thinking Critically 1. Calculate Suppose you start with one bacterium under

ideal conditions. If no bacteria die, compute the number ofbacteria present after 1 hour, 5 hours, and 10 hours.

2. Predict Trends What environmental factors might affecta bacterial population’s reproduction?

3. Error Analysis The above graph is an example of onegroup’s data.a. What error did they make in the y-axis of the graph? b. Redraw the graph correctly.

4. Infer An elephant reproduces once every four to six years.Why are elephants not likely to be used in laboratory pop-ulation studies?

Num

ber o

f or

gani

sms

Time in hours

0

32 728

1 072 431 104

0 5 10

Reproduction Rate of Bacteria

0091-0099 C04S1 BDOL-829900 8/3/04 7:12 PM Page 95

Slow life-history organisms reproduceand mature slowly, and are long-lived.They maintain population sizes at ornear carrying capacity.

Density factors andpopulation growth

Recall that limiting factors arebiotic or abiotic factors that deter-mine whether or not an organism canlive in a particular environment.Limited food supply, space, chemicalsproduced by plants themselves,extreme temperatures, and evenstorms affect populations.

How organisms are dispersed canalso be important. Figure 4.6 showsthree patterns of dispersal: random,clumped, and uniform.

Ecologists have identified two kindsof limiting factors that are related to dispersal: density-dependent and density-independent factors. Popu-lation density describes the number ofindividuals in a given area.

96 POPULATION BIOLOGY(tl)Envision/B.W. Hoffman, (tr)Matt Meadows, (b)John Kieffer/Peter Arnold, Inc.

Figure 4.6Organisms disperse in a variety of ways:random (A), clumped (B), and uniform (C).Clumping may be the most common. Uni-form dispersal may be the least commonalthough it is visible with birds on a wireand among creosote bushes in a desert.The orderly planting of crops, such as rows of corn, is not a natural dispersal. AA

BB

CC

Clumped

Uniform

Random

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Density-dependent factors in-clude disease, competition, predators,parasites, and food. These factors havean increasing effect as the populationincreases. Disease, for example, canspread more quickly in a populationwith members that live close together.In crops such as corn or soybeans in which large numbers of the sameplant are grown together, a disease can spread rapidly throughout thewhole crop. In less dense populations,fewer individuals may be affected.Disease is also a factor in human pop-ulations. The presence of HIV/AIDSin many of the world’s populations isconsidered by some scientists to be alimiting factor in the growth of thosepopulations.

Density-independent factors canaffect populations, regardless of theirdensity. Most density-independent factors are abiotic factors, such as vol-canic eruptions, temperature, storms,floods, drought, chemical pesticides,and major habitat disruption, such asthat shown in Figure 4.7. Although allpopulations can be affected by thesefactors, the most vulnerable appear tobe small organisms with large popula-tions, such as insects. No matter howmany earthworms live in a field, theywill drown if it floods. It doesn’t matterif there are many or few mosquitoes—a severe winter will kill the adults ofmost species.

Organism InteractionsLimit Population Size

Population sizes are limited not onlyby abiotic factors, but also are con-trolled by various interactions amongorganisms that share a community.

Predation affects population sizeA barn owl kills and eats a mouse. A

swarm of locusts eats and destroys acresof lettuce on a farm. When the browntree snake was introduced in Guam, an

island in the North Pacific, there wereno native predators for the snake.Consequently, it freely preyed on thenative birds of the island. These exam-ples demonstrate how predation canaffect population sizes in both minorand major ways. When a predator con-sumes prey on a large enough scale (asin the case of the brown tree snake), itcan have a drastic effect on the size ofthe prey population. For this reason,predation can be a limiting factor onpopulation size.

Populations of predators and theirprey are known to experience cycles orchanges in their numbers over periodsof time. Under controlled conditions,such as in a laboratory, predator-preyrelationships often show a predictablecycle of population increases anddecreases over time. In nature, thesecycles have also been observed. Oneclassic example of this has been demon-strated in Figure 4.8 on the next page,which shows a graph of 90 years of dataabout the populations of the Canadianlynx and the snowshoe hare. A memberof the cat family, the lynx stalks, attacks,and eats the snowshoe hare as a pri-mary source of food.

4.1 POPULATION DYNAMICS 97Robert McLeroy/San Antonio Express/CORBIS Sygma

Figure 4.7A flood of catastrophicproportions, such as thisflood in San Antonio,Texas in the summer of2002, can be a limitingfactor, especially forhuman populations.Among other things,flooding affects drink-ing water and sewagesystems. These are limit-ing factors for modernurban life. Infer Whatelse in this photo-graph might beaffected by flooding?

0091-0099 C04S1 BDOL-829900 8/3/04 7:08 PM Page 97

The data in Figure 4.8 show the lynxand hare populations appear to rise andfall fairly closely in a 10-year cycle.When the hare population increases,there is more food for the lynx popula-tion, and the lynx population increases.When the lynx population rises, preda-tion increases, and the hare populationthen declines. With fewer hares avail-able for food, the lynx population thendeclines. Then, with fewer predators,the hare population increases, and thecycle continues. This example showshow predator populations can affect the size of the prey populations. At thesame time, prey populations affect the size of the predator populations. Asthe snowshoe hare’s food supply ofgrasses and herbs dwindles during thefall and winter months, the hare popu-lation decreases. Because there are nowfewer hares to hunt, the lynx populationalso decreases. With the return ofspring, the hare’s food supply and itspopulation recover. This leads to morehares, allowing the lynx population toincrease as well.

Usually, in prey populations, theyoung, old, or injured members are caught. Predation increases thechance that resources will be availablefor the remaining individuals in aprey population.

Competition within a populationThe hare and the lynx belong to

different populations. What happenswhen organisms within the same population compete for resources?When population numbers are low,resources can build up and becomeplentiful. Then, as these resources are used, the population increases in size and competition for resourcessuch as food, water, and territoryagain increases significantly. Compe-tition is a density-dependent factor.When only a few individuals competefor resources, no problem arises.When a population increases to thepoint at which demand for resourcesexceeds the supply, the populationsize decreases.

98 POPULATION BIOLOGY

(l)S

teph

en J

.Kra

sem

ann/

Pet

er A

rnol

d, I

nc.,

(r)R

ober

t Wils

on/A

nim

als

Ani

mal

s

Num

ber o

f org

anis

ms

(in th

ousa

nds)

160180

80

204060

100120140

01845 1855

LynxHare

1865 1875 1885 1895 1905 1915 1925 1935Time (in years)

Lynx and Hare Pelts Sold to the Hudson‘s Bay Company*

* Data from 1844 through1904 reflects actual peltscounted. Data from 1905through 1935 is based onanswers to a questionnaire.

Figure 4.8The data in this graphreflect the number ofhare and lynx pelts soldto the Hudson’s BayCompany in northernCanada from 1845through 1935. Noticethat as the number ofhares increased, so didthe number of lynx.

0091-0099 C04S1 BDOL-829900 8/3/04 7:08 PM Page 98

The effects of crowding and stress

When populations of certain organ-isms become crowded, individuals mayexhibit symptoms of stress. The fac-tors that create stress are not wellunderstood, but the effects have beendocumented from experiments andobservations of populations of severalorganisms including fish, deer, rabbits,and rats as shown in Figure 4.9.

As populations increase in size in environments that cannot supportincreased numbers, individual animalscan exhibit a variety of stress symptoms.These include aggression, decrease inparental care, decreased fertility, anddecreased resistance to disease. All ofthese symptoms can have negativeeffects on a population. They becomelimiting factors for growth and keeppopulations below carrying capacity.

Understanding Main Ideas1. Explain and illustrate how the long-term survival

of a species depends on resources that may belimited from time to time.

2. Compare short and long life-history patterns.

3. Describe how density-dependent and density-independent factors regulate population growth.

4. Describe the population growth curve of houseflies.

Thinking Critically5. How can a density-dependent factor, such as

a food supply, affect the carrying capacity of ahabitat?

6. Get the Big Picture Graph the following sea-sonal population data for an organism shown inthe table below and analyze whether the organ-ism has a population growth pattern closer to arapid or slow life-history pattern. For more help,refer to Get the Big Picture in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

4.1 POPULATION DYNAMICS 99(l)Marcie Griffen/Animals Animals, (r)KS Studios

Seasonal Population Data

Year Spring Summer Autumn Winter

1995 564 14 598 25 762 127

1996 750 16 422 42 511 102

1997 365 14 106 36 562 136

Figure 4.9Stress caused by overcrowding in a rat popula-tion can limit population size. At birth, crowdingmay serve a positive purpose for keeping youngwarm (A). Older groups (B), of certain organismswhen overcrowded, may fight and kill eachother. They may reproduce less, and they maystop caring for offspring.

AA

BB

ca.bdol.glencoe.com/self_check_quiz

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4.2SECTION PREVIEWObjectivesIdentify how thebirthrate and death rateaffect the rate at which apopulation changes.Compare the age struc-ture of rapidly growing,slow-growing, and no-growth countries.Explain the relationshipbetween a population andthe environment.

Review Vocabularylimiting factor: factors that

affect an organism’sability to survive in itsenvironment (p. 65)

New Vocabularydemography birthrate death ratedoubling timeage structure

100 POPULATION BIOLOGY

Human Population

Tom Van Sant/Geosphere Project, Santa Monica, CA/SPL/Photo Researchers

1st billion

1800

2nd billion

1930

3rd billion

1960

4th billion

1975

5th billion

1987

6th billion

1999

Figure 4.10It is estimated that it tookfrom the dawn of human history to 1800 for the world’s population to reach 1 billionpeople. Today, there are morethan 6 billion people in theworld, and scientists estimatethat by the year 2050, therewill be more than 9 billionpeople on Earth.

World Population In the United States, a census is taken every ten years. Among other

things, this information provides a picture of how many people there arein the United States, their economic condition, and where they live.Worldwide, the United Nations Population Division tracks similar infor-mation on all the countries of the world. One of the most useful pieces ofdata is the rate at which each country’s population is growing or declin-ing. These figures are the basis for demography (de MAH gra fee), thestudy of human population size, density and distribution, movement, andits birth and death rates.

What is the history of population growth for humans? Figure 4.10summarizes how world human population has grown since 1800. Thegraph indicates that until the 1800s, human population growth remainedfairly slow. Since the 1930s, world population has grown rapidly, reach-ing 6 billion in 1999. In 2002, the human population was growing at arate of more than 80 million people per year.

Keeping TrackFinding the Main Idea Can you imagine having the responsibility of keeping track of how many people there are in the world? Why is it important to know how many people there are and where they are located? These are topics discussed in this section. To help you keep track of the information, it might be a good idea to find the main idea discussed in each paragraph.Summarize As you study, look at each bold head. Then read the information under it care-fully. After each paragraph, write one sentence that summarizes the main idea of the paragraph.

Standard 1e Students will solve scientific problems by using quadraticequations and simple trigonometric, exponential, and logarithmic functions.

California Standards

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4.2 HUMAN POPULATION 101

Human population growthWhat factors affect growth of

human population? In Section 1 of this chapter, bacteria and housefly pop-ulations were shown to continue togrow so long as they had sufficientresources. Human population growthis different because humans can con-sciously change their environment.During the past century, humans haveeradicated diseases such as smallpox.They have developed methods for pro-ducing more food. Infant mortality ratehas decreased and technological devel-opments have improved the delivery ofclean water. When these factors areaccounted for, people live longer andare able to produce offspring that livelong enough to produce offspring,hence, a population grows.

Calculating growth rate There are a number of factors that

determine population growth rate.These are births, deaths, immigrationand emigration. Birthrate is the num-ber of live births per 1000 population ina given year. Death rate is the numberof deaths per 1000 population in a givenyear. Movement of individuals into apopulation is immigration. Movementout of a population is emigration. Youcan calculate a country’s populationgrowth rate with a formula that takesthese four factors into account:

(Birthrate � Immigration rate) � (Death rate � Emigration rate) � Population Growth Rate (PGR)

For convenience, and because immi-gration and emigration rates are notalways accurate, this formula is oftenstated as:

Birthrate � Death rate � Population Growth Rate (PGR)

If the birthrate of a population equalsits death rate, then the populationgrowth rate is zero. If the rate is zero,that doesn’t mean that the population

isn’t changing. Rather, it means thatnew individuals enter the population(by birth and immigration) at the samerate that individuals are leaving (bydeath and emigration) the population.The population is changing, but it isstable. If the PGR is above zero,more new individuals are enteringthe population than are leaving, sothe population is growing.

Make and Use GraphsHow is world population changing? Total world populationand the rate at which it is growing are predicted to change inthe next 50 years. Plotting this information on a graph can pro-vide a visual that tells you how it is predicted to change.

The table above contains figures from the U.S. Census Bureauand the United Nations Population Bureau that predict worldpopulation change through 2050. Graph the data and answerthe questions that follow.

Thinking Critically1. Analyze Trends from Data Study your graph and

choose the term that best describes the trend that thegraph illustrates: Rising, leveling off, or declining.Explain your choice.

2. Infer Based on the data in the table, what can you inferhappened to the population growth rate after 1985?

3. Predict Trends from Data Based on the data predicted for2010 through 2050, how would you describe world popula-tion? Growing? Declining? Stable? Explain your choice.

1950 2 555 360 972

1960 3 039 669 330

1970 3 708 067 105

1980 4 454 607 332

1985 4 850 118 838

1990 5 275 407 789

1995 5 685 286 921

2000 6 078 684 329

2005 6 448 684 573

2010 6 812 009 338

2015 7 171 736 193

2020 7 515 218 898

2025 7 834 028 430

2030 8 127 277 506

2035 8 397 941 844

2040 8 646 671 023

2045 8 874 116 015

2050 9 078 850 714

Total Midyear Population for the World 1950–2050

Year Population Year Population

0100-0109 C04S2 BDOL-829900 8/4/04 8:41 AM Page 101

A PGR can also be less than zero.In 2002, the population growth ratefor Europe was negative (�0.1 per-cent) as fewer individuals are enter-ing the population than are leaving.

The effect of a positive growth rate

If the world population growth rate in the year 1995 were 1.7 percentand had dropped to 1.3 percent in2001, the population growth ratewould have become lower, but worldpopulation would have continued to

grow, just at a slower rate. In otherwords, unless the growth rate becomesnegative, the population continues togrow, but just not as rapidly as it didbefore.

Doubling timeAnother quantitative factor that

demographers look at is the doublingtime of a population. Doubling time isthe time needed for a population todouble in size. The time it takes for apopulation to double varies dependingon the current population and growthrate. A slow or negative growth ratemeans that it will take a country’s pop-ulation a long time to double in size, ifever. A rapid growth rate indicates thata country’s population will double in ashorter time. A country that has a slowdoubling time is sometimes catego-rized as a developed country. One witha rapid doubling time may be referredto as a developing country. Doublingtime can be calculated for the world, acountry, or even a smaller region, suchas a city. Learn how to calculate dou-bling time in the MiniLab on this page.

Age structureHave you ever filled out a survey?

Often, one of the questions is aboutage. Are you between the ages of 10 and14? 15 and 19? 20 and 24? The surveyis trying to pinpoint where you are inthe age structure of the population. Agestructure refers to the proportions ofthe population that are in different agelevels. Based on information from pop-ulation counts, an age structure graphhas been constructed for every countryin the world. Look at the age structuregraphs in Figure 4.11. An age struc-ture graph can tell you approximatelyhow many males and females there arein a population, and how many peoplethere are at each age level. Rapidlygrowing countries have age structureswith a wide base because a large

102 POPULATION BIOLOGY102 POPULATION BIOLOGY

�Doubling time (in years) 70annual percent growth rate

Use NumbersDoubling Time The time needed for any population to dou-ble its size is known as its “doubling time.” For example, if apopulation grows slowly, its doubling time will be long. If it isgrowing rapidly, its doubling time will be short.

Procedure! The following formula is used to calculate a population’s

doubling time:

@ Copy the data table below. # Complete the table by calculating the doubling time of

human populations for the listed geographic regions.

Analysis 1. Analyze Trends from Data Which region has the fastest

doubling time? Slowest doubling time?2. Predict Trends from Data What are some ecological

implications for an area with a fast doubling time?

Data Table

Geographic Annual Percent DoublingRegion Growth Rate Time

A 2.4

B 1.7

C 1.4

D 0.5

E –0.1

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4.2 HUMAN POPULATION 103

Population Distribution Per Age Range for Several Countries

4 2 0 2 4 8 6 4 2 0 2 4 6 8 6 4 2 012 10 8 2 4 6 8 10 120–45–9

10–1415–1920–2425–2930–3435–3940–4445–4950–5455–5960–6465–6970–7475–79

80+

Population (percent of total for each country)

Age

Stable growth Rapid growth Slow growth

MaleFemaleReproductiveyears

Figure 4.11Notice that in a rapidgrowth country thelarge number of individ-uals in the “Under 5Through 14 years” willadd significantly to thepopulation when theyreach age 15. Popula-tions that are not grow-ing or are stable havean almost even distribu-tion of ages among thepopulation.

percentage of the population is madeup of children and teenagers. If thepercentage of people in each age cat-egory is fairly equal, the populationis stable.

Ecology and growthThe needs of populations differ

greatly throughout the world. Somecountries are concerned about provid-ing the most basic needs for theirgrowing population. Other, more sta-ble growth populations are concernedabout maintaining the healthy condi-tions that they already have.

What do populations need? Thinkabout the resources that humans

depend upon every day. Some of theseresources might be uncontaminatedwater for drinking and agriculture, ade-quate sewage facilities, and the ability toprovide food for a growing population.

Sometimes, a population growsmore rapidly than the availableresources can handle. Resources thatare needed for life, such as food andwater, become scarce or contami-nated. The amount of waste producedby a population becomes difficult todispose of properly. These conditionscan lead to stress on current resourcesand contribute to the spread of dis-eases that affect the stability of humanpopulations both now and to come.

ca.bdol.glencoe.com/self_check_quiz

Understanding Main Ideas1. What characteristics of populations do demogra-

phers study? Why?

2. How do birthrate and death rate each affect thegrowth of a population?

3. What clues can an age structure graph provideabout the future of a country’s population growth?

4. Explain the relationship between a growing popu-lation and the environment.

Thinking Critically5. Suggest reasons why the lack of available clean

water could be a limiting factor for a country’spopulation.

6. Make and Use Graphs Construct a bar graphshowing the age structure of Kenya using thefollowing data: pre-reproductive years (0–14)—42percent; reproductive years (15–44)—39 percent;post-reproductive years (45–85�)—19 percent. Formore help, refer to Make and Use Graphs in theSkill Handbook.

SKILL REVIEWSKILL REVIEW

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Before You BeginIn the field, scientistsdetermine the number ofanimals in a large popula-tion by sampling. Theytrap and mark a few ani-mals in a specified area.The animals are releasedand the traps are reset.Scientists wait a period oftime before they retrap.This time allows organismsto mix randomly into thepopulation again. Amongthe animals caught thesecond time, some willalready be marked andsome will be unmarked.Scientists then calculate thetotal population based onthe ratio of marked animalsto unmarked animals.

104

How can you determinethe size of an animalpopulation?

ProblemHow can you model a field-measuring technique to determine the size of an animal population?

ObjectivesIn this BioLab, you will:■ Model, using a simulation, a procedure used to measure an

animal population.■ Collect data on a modeled animal population.■ Calculate the size of a modeled animal population.

Materialspaper bag containing beans permanent marker calculator (optional) (dark color)

Safety PrecautionsCAUTION: Always wear goggles in the lab. Wash hands withsoap and water after working with plant material and afterclean up.

Skill HandbookIf you need help with this lab, refer to the Skill Handbook.

1. Copy the data table.

2. Reach into your bag and remove 20 beans.

3. Use the marker to color these beans. They representcaught and marked animals.

4. When the ink has dried, return the beans to the bag.

5. Shake the bag. Without looking into the bag,reach in and remove 30 beans.

6. Record the number of marked beans (recaughtand marked) and the number of unmarkedbeans (caught and unmarked) in your datatable as trial 1.

7. Return all the beans to the bag.

PROCEDUREPROCEDURE

PREPARATIONPREPARATION

Aaron Haupt

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ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Think Critically Explain why this type of activity is best done as a simulation.

2. Compare the calculated tothe actual population size. Explain why theymay not agree exactly. What changes to theprocedure would improve the accuracy of the activity?

3. Make Inferences Explain why this tech-nique is used more often with animals thanwith plants when calculating population size.

4. Make Predictions Assume you were doingthis experiment with living animals. Whatwould you be doing in step 2? Step 3? Step 5?

ERROR ANALYSIS

4.2 HUMAN POPULATION 105

Calculated population size = ______

Actual population size = ______

TrialTotal Caught

1

2

3

4

5

Averages

30

30

30

30

30

30

Number Caught With Marks

Number Caught Without Marks

Data Table

8. Repeat steps 5 to 7 four more times for trials 2 to 5.9. Calculate averages for each of the columns.

10. Using average values, calculate the original size of thebean population in the bag by using the following formula:M � number initially markedCwM � average number caught with marksCw/oM � average number caught without marks

11. Record the calculated population size in the data table.12. To verify the actual population size, count all the beans in

the bag and record this value in the data table.13. Make wise choices as to how you

will dispose of the beans. Can some be recycled?CLEANUP AND DISPOSAL

Field Investigation Assume that you are afield biologist on an island. Plan investigativeprocedures, including selecting equipment, todetermine the deer population on the island.

Web Links To find out more aboutecology field research, visit ca.bdol.glencoe.com/ecology

�Calculated

Population SizeM � (CwM � Cw/oM)

CwM

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106 POPULATION BIOLOGYAaron Haupt

Polymers forPeople

Polymerization is the process in which singlemolecules of a substance are joined chemi-

cally to form long chains called polymers. Onepolymer that has had a great affect on modern lifeis polystyrene. Because of polystyrene, we havenumerous disposable plastic items in our lives,from plastic grocery bags to jewel cases for CDs.

Polystyrene, in the form of foamed plastic, isabout 98 percent air. It is made by blowing tinyair-containing holes, called cells, into a polymer.In the beginning, chlorofluorocarbons, or CFCs,were used to make the cells. In the 1980s, CFCswere outlawed for this and other processes.Since then, most foamed polystyrene has beenmade using pentane as the blowing agent.

Polystyrene products Foam beverage cupsand plates, plastic utensils, some packaging“peanuts,” insulation, and many disposablepieces of medical equipment are made frompolystyrene. In the food service industry, poly-styrene products keep hot foods hot and coldfoods cold. These products are popular for theirhealth safety because they are used once andthrown away. This feature reduces the chance ofcontamination and transmission of disease.

Because of all of its positive characteristics,polystyrene is used extensively. However, theedges of highways and our landfills reflect itswidespread use by a growing population.Foamed plastic, as it currently exists, is notbiodegradable within a reasonable amount oftime. How can the problems created by massdisposal of items made from polystyrene beavoided? Is there a way to make a biodegradableplastic bag, a CD case, toy, or toothbrush?

Biodegradable products For something tobe biodegradable, it has to be able to be broken

down into simpler components by decomposers.Polystyrene can be broken down, but it takes along time for that to happen. Is there anythingthat can be broken down more quickly?

Corn into plastic Have you ever worn a shirtmade out of corn resin? Research for more envi-ronmentally-friendly substances is an on-goingproject. Since the 1980s, manufacturingprocesses for turning corn starch and corn fibersinto useful products have become a reality.Corn-based packing “peanuts” have been devel-oped. Molded bottles are being manufacturedfor use in short-term shelf products such as milk.The hope is that these products and others willconserve fossil fuels, be quickly biodegradable,and therefore, more environmentally friendly.Markets for corn-based polymers are similar tothose for petroleum-based polystyrene prod-ucts—leaf and lawn bags, food packaging, andtextiles for clothing. Research is ongoing withregard to how well microorganisms break downthe materials.

Research Find out about alternative biodegrad-able materials being developed for the purpose ofconserving fossil fuels and for making landfillsmore useful. Evaluate the impact of polymers bysurveying your own home for them. Compare yourfindings with those of your classmates.

To find out more about plastics and biodegradable products and their role in the environment, visitca.bdol.glencoe.com/chemistry

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Section 4.1Key Concepts■ Populations of some organisms do not

exhibit linear growth. If there is nothing tostop or slow growth, a population’s growthappears as a J-shaped curve on a graph.

■ Populations grow slowly at first, then morerapidly as more and more individuals beginto reproduce.

■ Under normal conditions, with limitingfactors, populations show an S-shapedcurve as they approach the carrying capac-ity of the environment where they live.

■ If a population overshoots the environ-ment’s carrying capacity, deaths exceedbirths and the total population falls belowthe environment’s carrying capacity. Thenumber of individuals will fluctuate aboveand below the carrying capacity.

■ Density-dependent factors and density-independent factors affect populationgrowth. Density-dependent factors includedisease, competition for space, water, andfood supply. Density-independent factors are volcanic eruptions and changesin climate that result in catastrophic inci-dents such as floods, drought, hurricanes,or tornadoes.

Vocabularycarrying capacity (p. 93)density-dependent

factor (p. 97)density-independent

factor (p. 97)exponential growth

(p. 93)life-history pattern

(p. 93)

PopulationDynamics

Key Concepts■ Demography is the study of population

characteristics such as growth rate, agestructure, and movement of individuals.

■ Birthrate, death rate, immigration, emigra-tion, doubling time, and age structures differconsiderably among different countries.There are uneven population growth pat-terns throughout the world.

Vocabularyage structure (p. 102)birthrate (p. 101)death rate (p. 101)demography (p. 100)doubling time (p. 102)

HumanPopulation

STUDY GUIDESTUDY GUIDE

CHAPTER 4 ASSESSMENT 107Thomas Kitchin/Tom Stack & Associates

Section 4.2

To help you review pop-ulation biology, use the OrganizationalStudy Fold on page 91.

ca.bdol.glencoe.com/vocabulary_puzzlemaker

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108 CHAPTER 4 ASSESSMENT ca.bdol.glencoe.com/chapter_test

Review the Chapter 4 vocabulary words listed inthe Study Guide on page 107. Distinguishbetween the vocabulary words in each pair.

1. birthrate—death rate 2. density-dependent factor—

density-independent factor 3. life-history pattern—age structure4. population—demography5. limiting factor—carrying capacity

6. Describe what is happening to the growth ofthe population shown at interval 3 in thediagram below. A. slow growthB. exponential or rapid growthC. slowing growth reaching carrying capacityD. population reaching equilibrium near

carrying capacity

7. Which organisms would be most affected by density-independent factors?A. catsB. humansC. housefliesD. deer

8. When plotted on a graph, a population offield mice over time shows a J-shaped curve.This indicates that ________.A. the population is decreasingB. predators of the mice are increasingC. there may be no predatorsD. food supply is low

9. When populations increase, resource deple-tion may bring about ________. A. exponential growthB. straight-line growthC. increased competitionD. decreased competition

10. Open Ended A population of animals showsa sudden decline and then recovers. Usingecological principles, discuss two specificreasons why this might occur.

11. Open Ended Give at least two reasons why itwould be important for the planning board ofa city to know the city’s doubling time.

12. Open Ended For what reasons would aschool board need to refer to an age struc-ture chart of the local community whenplanning a five year budget?

13. Analyze Using the diagram above, identifythe factor that would cause more trees inArea B to be affected by an invasion of disease-carrying insects than in Area A.Explain your choice.

14. Infer Are predators a density-dependent or density-independent limiting factor for thepopulation growth of their prey? Explain.

15. Infer Why are short life-history species,such as mosquitoes and some weeds, suc-cessful, even though they often experiencemassive population declines?

Area A Area B

1 2 3 4

Pop

ula

tio

n s

ize

Time

Carrying capacity

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16. Every 10 yearsthe United States is required by law to com-plete a census. Visit to find out about the most recent census.Pretend that you are a demographer foryour state. What change, if any, occurred

in the population of your state from 1990 to 2000? Display these changes on a map. Determine if your state’s popu-lation is growing, declining, or has reached stability. Research and explain your choice.

REAL WORLD BIOCHALLENGE

CHAPTER 4 ASSESSMENT 109

Nu

mb

er o

f co

lon

ies

Temperature test runs

Number of Colonies of a Bacterium Grown at Different Temperatures

1 2 3 4

706050403020100

Nu

mb

ero

f d

eer

Time

Deer Population Over Time

A

B

ca.bdol.glencoe.com

ca.bdol.glencoe.com/standardized_test

Multiple ChoiceUse the graph below to answer questions 17 and 18.

17. A bacterial species was grown at differenttemperatures represented in the graph aboveby cultures 1 through 4. From the graph,identify the culture for which temperaturewas the greatest limiting factor.A. 1B. 2C. 3D. 4

18. Which culture showed the greatest growthrate?A. 1 C. 3B. 2 D. 4

Use the following diagram to answer questions 19 and 20.

19. The dotted line in the graph above representsthe ________ for the deer population.A. death rate C. carrying capacityB. birthrate D. age structure

20. The solid curve in the graph from point Ato point B indicates that ________.A. more deer are dying than being bornB. more deer are being born than are dyingC. there are not enough predatorsD. no limiting factors are at work

Constructed Response/Grid InRecord your answers on your answer document.

21. Open Ended A small group of mice invaded a new habitat with unlimited resources and theirpopulation grew rapidly. A flood then swept through the habitat and three quarters of the micewere lost. Two months later, the population was increasing again. What role did the flood playfor the mouse population? Draw a graph depicting the population history of this group.

22. Open Ended What is the relationship between a population and a species?23. Open Ended Water hyacinth populations double in 6 to 18 days. Introduced in the 1880s, popula-

tions of this plant have clogged major waterways in several states. No predators for it exist in theUnited States. Does this species have a J-shaped or an S-shaped growth pattern? Explain your choice.

6b

6c

6c

The assessed California standard appears next to the question.

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